My DS16 went through this from about 12-14. (6th-8th grade) When he wasn't going to school or doing homework he just wanted time to veg by himself. This did get better when he hit high school.
My son is naturally an introvert and in 6th was having anxiety problems that translated into social problems at school. Even after I got him professional help with these he still CHOOSE not to participate in outside activities. I believe when school was over he found it exhausting to have to keep that social face on and he just wanted time to relax. He still really wants time just to veg. but he now participates in Marching Band and the H.S. Robotics Club, and goes out with friends occasionally. Still not the most social kid in the school but he is active and involved in school.
What I did insist was he had to be involved something physical. But that physical activity doesn't need to be something formal and at 12 they are old enough to send them outside for a bike ride or run. In junior high my son did keep up a martial art in a club that was mostly adults.
Junior High is a a rough age socially and emotionally. They are growing fast and hormones really take a toll. Add to this that we are in the internet age and teenagers don't socialize in person as much as we did a generation ago.
